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Publication# Comportement structural des bétons de fibres ultra performants en traction dans des éléments composés

Abstract

Ultra high performance concretes (UHPFRC) are characterized by a dense matrix and a high fibre content. These materials exhibit exceptional mechanical and durability properties making them an ideal material for rehabilitating existing structures. The primary interest in UHPFRC focuses on their uniaxial tensile performance. When they applied in a cast in place overlay configuration, they provide increased rigidity to the global element and localized protection to the steel reinforcement embedded in the concrete core during the service by minimizing surface cracking and inhibiting the diffusion of aggressive agents. At the ultimate state, and under certain configurations UHPFRC significantly improves the element's carrying capacity. The overall goal of this research is to study the UHPFRC tensile behaviour (hardening and softening behaviours) of characteristic test specimen and to apply this knowledge in analyzing the structural response of various composite elements. The objectives related to this study are: To determine the factors influencing the UHPFRC uniaxial tensile test specimen behaviour To illustrate the importance of UHPFRC tensile hardening and softening responses in UHPFRC structural plates and UHPFRC-reinforced concrete composite beams. To determine the rupture mechanism and overall contribution a UHPFRC overlay offers to a composite UHPFRC-reinforced concrete slab loaded to failure in flexural/punching shear. To study the parameters influencing the UHPFRC uniaxial behaviour in tensile composite structural elements. The behaviour of specimen with different configurations and compositions were studied in uniaxial tensile tests. The variables significantly influencing the fibre orientation were isolated by studying the fibre orientation at localized cuts in the various specimen. These controlling variables include: mixture viscosity, casting method, casting direction, fibre aspect-ratio and element geometry. Furthermore, a meso-level model was developed to predict the uniaxial tensile behaviour and was validated against the experimental test results. With the help of this model, the influence of the main fibre characteristics (Vf, Lf, df) on the resultant fibre orientation and matrix quality were examined. This model has determined that the extent of hardening does not increase linearly with the quantity of fibres but rather follows an asymptotic curve converging to a maximum possible hardening. A second model was developed to randomly orient and place fibres. This model has helped to explain the uniaxial tensile response variability of different mixtures ranging from conventional Fibre Reinforced Concrete (FRC) to UHPFRC. A finite element program has been employed to study the influence the UHPFRC softening behaviour has on the composite structural beam and UHPFRC plate responses. For a flexural plate (length 500mm, width 200mm, height 30mm), the bearing capacity was significantly increased by the very pronounced UHPFRC softening behaviour. Additionally, the UHPFRC layer noticeably increased the rigidity of the composite beam and slab. For example the UHPFRC layer increased the slab bearing capacity by 40% in comparison to a purely reinforced concrete slab by bridging the punching shear mechanism with a UHPFRC membrane effect. The structural responses of the two element types were then further analyzed using a finite element analysis program. An inverse analysis methodology was employed to obtain the UHPFRC tensile material laws in respective composite elements. These findings documented that a similar materials exhibited significantly different responses when applied in different applications. This analysis documents that the UHPFRC layer in a slab closely follows the uniaxial tensile test specimen response, while the UHPFRC layer in a composite beam exhibits a significantly reduced performance. The primary reasons for these variances are: the relative fibre distribution, the interface rugosity, the internal residual stresses and the extent of the maximum stress zone. Recommendations are made concerning the implementation of UHPFRC and the influence the fibre aspect ratio and volume (Vf, Lf, df) imposes on the resulting fibre orientation in different applications. Finally, a method for determining the UHPFRC characteristic curve which is required to design a composite UHPFRC-reinforced concrete element is presented.

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With the occurrence of higher and more frequent axle loads on roads, in particular bridge deck slabs are more severely solicitated by fatigue loading. To avoid heavy interventions for strengthening of deck slabs, an improved building material is used, namely Ultra-High Performance Fibre Reinforced Concrete with steel rebars (reinforced UHPFRC = R-UHPFRC). By adding a thin (30 to 50 mm) layer of R-UHPFRC on top of the bridge deck slab, the required fatigue resistance and load carrying capacity may be restored and improved. In addition, the R-UHPFRC layer is waterproof which provides durability. This paper presents a model to describe the fatigue behaviour of reinforced concrete (RC) slab-like beams strengthened with R-UHPFRC leading to RU-RC beams. The model determines stress and deformation evolution in components of the RU-RC beam by considering decrease of UHPFRC stiffness due to fatigue. Comparison with available experimental results shows that the model can accurately represent the beam behaviour. Force distribution among components of the RU-RC beam reveals efficient fatigue resistant behaviour of the RU-RC beam, by which R-UHPFRC is demonstrated to be an effective element for fatigue strengthening of RC member.

The addition of a thin overlay of Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) to Reinforced Concrete (RC) members is an emerging technique to strengthen and protect existing structures and to design durable new structures. Combining UHPFRC with closely spaced, small-diameter steel rebars in Reinforced UHPFRC (R-UHPFRC) layers improves the UHPFRC's strain hardening behaviour. For reasons of practicality, R-UHPFRC layers are cast or glued (in the case of prefabricated elements) on top of RC members, thus changing the latter into R-UHPFRC - RC composite members. The high strength and deformation capacity of R-UHPFRC elements make them a suitable external flexural reinforcement for RC members over intermediate supports, e.g., bridge decks and slabs or beams in buildings. Over reinforcement of RC beams and slabs with tensile flexural reinforcement can result in their shear failure at either a lower resistance or deformation than the associated values for member failure in flexure. A comprehensive experimental program was conducted to study the flexure-shear behaviour of R-UHPFRC - RC composite beams. The program comprises two test series on cantilever beams and continuous beams. The test parameters include shear span-depth ratio (a/d), the amount of transverse reinforcement ( ρν), the amount of longitudinal reinforcement, and the strength and bond condition of the R-UHPFRC rebars. The experimental results reveal the different failure modes of R-UHPFRC - RC composite members and the contribution of the R-UHPFRC elements to the member resistance, ductility and capacity to redistribute the internal stress. It was shown that in R-UHPFRC - RC beams with ribbed rebars and a shear span to depth ratio greater than 2.5 the stresses are carried by beam action. Depending on the degree of longitudinal reinforcement, all but two of the beams with 3.0≤a/d≤3.4 and ρν≤0.17 had a flexure-shear failure; the rest failed in flexure. The flexure-shear failure of the composite beams was at an approximately equal rotation level as their RC reference beam but at a resistance 2.3 times that of the RC beam. This is due to (1) the debonding interface zone between the elements that allows the R-UHPFRC - RC beams to rotate more freely and (2) the out-of-plane resistance of the R-UHPFRC element that contributes to the shear resistance. The internal flow of forces and the structural response of composite members strongly depend on the bond condition between the R-UHPFRC and RC, the UHPFRC and its rebars, as well as the concrete and its rebars. Cracking of the concrete along the interface zone causes bond reduction, i.e., softening of the shear connection, between the two elements. In presence of high shear stresses and diagonal flexure-shear cracks, interface zone softening is observed between the elements prior to the maximum resistance, while UHPFRC is strain hardening. The cause of this softening behaviour is the prying action due to the relative rotational movement of the RC rigid bodies separated by the flexure-shear cracks. Static and kinematic solutions of the theory of plasticity for RC beams are extended to predict the collapse load of R-UHPFRC - RC composite beams at the ultimate limit state. A mechanical model for predicting the structural response of composite beams is proposed. In combination with truss models, the concept of an R-UHPFRC - RC plastic hinge is introduced to calculate the force-displacement response of composite beams. The failure criterion based on the collapse mechanisms (kinematic solutions) sets the limit of the force-displacement response. The model is corroborated by the experimental results. This model provides a tool for analysis of RC members reinforced with an added tensile R-UHPFRC element.

Ultra-High Performance Fibre-Reinforced Concretes (UHPFRC) have high mechanical strengths (fU,c > 150 MPa, fU,t > 6 MPa) and exhibit quasi-strain hardening in tension. Their very low permeability prevents the ingress of detrimental substances. In composite structural elements formed of normal strength reinforced concrete and Advanced Cementitious Materials (ACM), UHPFRC offer a high potential in view of the load carrying and protection function of the ACM layer. The objectives of the study described in this thesis are to determine the performance and structural behaviour of composite "UHPFRC-concrete" elements in bending. Towards this end, the current knowledge of UHPFRC properties is to be extended and modelling tools are to be developed in order to predict the structural behaviour of such composite elements and to make recommendations for their design. The experimental program is performed in order to characterize the UHPFRC and determine the structural behaviour of composite "UHPFRC-concrete" elements through 15 full-scale beam bending tests. The material tests focus on UHPFRC early age behaviour and on the determination of its outstanding tensile properties with an original uniaxial tensile test. They show that the UHPFRC properties become virtually constant after 90 days. The time-dependent behaviour of the composite beams is investigated during 11 weeks starting from the casting of the UHPFRC layer. After these long-term tests, the beams are subjected to bending until failure with the UHPFRC layer in tension. The parameters are the thickness of the UHPFRC layer, the presence of rebar in the UHPFRC and the static system. The time-dependent behaviour of composite "UHPFRC-concrete" members is investigated focusing on early age by means of test results and an existing numerical model. Using the results of the material tests as input, the numerical model is able to predict the behaviour of composite "UHPFRC-concrete" elements with the exception of self-desiccation of the UHPFRC. However, the deformations of composite "UHPFRC-concrete" members are correctly modelled by introducing autogenous shrinkage directly as volumetric deformation in the model. The structural response of the composite members under bending with the UHPFRC layer in tension is investigated with an original analytical model, which is an extension of the classical bending model for reinforced concrete. The influences of cross-section geometry, the UHPFRC tensile properties, the compressive strength of the substrate and the type of reinforcement are studied. This study demonstrates that the use of UHPFRC enhances the performance of composite "UHPFRC- concrete" elements in terms of resistance and stiffness. Furthermore, durability is extended due to the low permeability and tensile strain hardening properties of UHPFRC. The incorporation of rebar in the UHPFRC layer leads to a further increase in resistance and stiffness of the composite element and to a higher apparent magnitude of hardening in the UHPFRC. The investigated composite elements show monolithic behaviour under service conditions. The time-dependent behaviour is mainly controlled by autogenous shrinkage of the UHPFRC, which may induce a few evenly distributed small-width macrocracks in the case of statically indeterminate systems, thin UHPFRC layers (≤ 1 cm) or high magnitudes of autogenous shrinkage (≥ 750 µm/m at 28 days). The results show that the magnitude of autogenous shrinkage should not exceed 1000 µm/m (at 28 days) in order to avoid debonding and extensive formation of distributed macrocracks. Finally, three basic composite "UHPFRC-concrete" element configurations are proposed: Configuration P is designed for the protection function and consists of a thin UHPFRC layer. Configuration PR is proposed for existing elements with strongly deteriorated rebar and for new construction. It consists of an UHPFRC layer with reinforcement, and assumes no reinforcement in the concrete layer near the interface zone. Configuration R is proposed for existing structures requiring an enhancement of the structural behaviour. It is made of an UHPFRC layer with reinforcement, and assumes that there is reinforcement in the concrete layer near the interface zone.